Chapter 8: Sympathetic Neuropharmacology and Adrenergic Agonists

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Welcome back to The Deep Dive.

I'm your host and today we are tackling something that I think every student of the human body eventually has to face down.

We are cracking open Brenner and Stephens's pharmacology sixth edition and we are turning specifically to chapter eight.

Sympathetic neuropharmacology and adrenergic agonists.

It sounds intimidating right out of the gate doesn't it?

I mean you see those words and you just picture endless charts, chemical structures and just a headache waiting to happen.

It certainly has that reputation.

It's well it's dense.

Yeah.

There's no getting around that but I would argue this is actually one of the most vital chapters in the entire book.

Really?

Why is that?

Well if you don't understand this chapter you can't really understand how we keep people alive in the ICU or you know how we treat asthma or even how a simple nasal spray works.

Because this is the machinery of fight or flight.

It is.

This is the operating system for survival.

When you get scared when you're running for a bus or when your body goes into shock from an infection this is the system that wakes up and the drugs we are going to talk about the adrenergic agonists are basically us trying to like hotwire that system to get a very specific result.

Okay so the mission for today is to break this down so that it's not just a list of drugs to memorize.

We want to build a mental map.

We want to understand the factory that makes the chemicals, the dashboard of receptors that receives the signals, and then finally the drugs that press all the buttons.

And we are going to stick strictly to the text of chapter eight.

We aren't wandering off into other theories or clinical practices not mentioned here.

We are going to walk through the logic exactly as Brenner and Stevens presented.

Why that approach specifically?

Because they lay it out in a very specific build from the ground up way.

If you follow their path it clicks.

If you jump around you get lost.

All right I'm sold.

Let's start at the very beginning.

Before we can even think about drugs we have to talk about the body's own natural chemistry.

Section one of our outline is the life cycle of a neurotransmitter.

Right so if you have to look open we are looking at figure 8 .1.

This is a diagram of a sympathetic nerve terminal.

The very end of the neuron right where it meets the target tissue like a muscle or a gland.

I always picture this like a little factory floor at the end of the nerve.

That's a perfect analogy.

It is a factory and the product this factory is responsible for manufacturing is norepinephrine.

Okay.

But you can't make a product without raw materials.

Well of course not.

So what's the raw material here?

An amino acid called tyrosine.

It doesn't just appear.

It enters the nerve terminal from the outside transported across the cell membrane.

So the delivery truck drops it off at the loading dock.

Exactly.

Once it's inside the factory the assembly line begins.

Okay walk us through the steps.

Tyrosine is in the building.

What happens first?

It meets the first manager on the floor which is an enzyme called tyrosine hydroxylase.

This enzyme takes tyrosine and converts it into a compound called dopa.

Just dopa?

Just dopa for now.

Oh.

And we have to pause here because this is arguably the most important step in the entire process.

Wait why?

It's just step one.

Seems pretty basic.

Because tyrosine hydroxylase is the rate limiting enzyme.

Meaning it sets the speed limit.

It's the bottleneck.

Exactly the bottleneck.

That's the perfect word for it.

The factory can only produce norepinephrine as fast as this one enzyme can work.

So it's a safety mechanism.

It's a biological safety mechanism.

A crucial one.

You don't want your body to accidentally flood your system with adrenaline just because you ate a steak rich in tyrosine you know.

The body puts a governor on the engine right here at the start.

So if this enzyme is slow the whole line is slow.

If it's fast we get a surge.

Correct.

And the body regulates this enzyme very tightly.

For example if there's already a ton of norepinephrine floating around it actually feeds back and inhibits this enzyme.

It says hey we have enough stock shut down the line for a bit.

A negative feedback loop.

Precisely.

So naturally there must be a drug that targets this specific manager.

There is.

It's called meterosine and it is a competitive inhibitor of tyrosine hydroxylase.

Competitive inhibitor.

So it basically steps in front of the enzyme and stops it from doing its job.

It gums up the works right at the start.

So shuts down the production of adrenaline and norepinephrine at the source.

Yes.

Now why would we want to do that?

I mean it feels like we generally want our fight or flight system to be available when we need it.

In a normal person yes.

You're absolutely right.

Meterosine is not a drug you take for a headache.

It is used for a very specific very dangerous condition called a pheochromocytoma.

Pheochromocytoma.

Okay break that down for me.

It's a tumor usually of the

all the safety rules.

It just pumps out massive uncontrolled amounts of epinephrine and norepinephrine.

So the patient is just constantly in a state of maximum overdrive.

Constantly.

We're talking severe life -threatening hypertension.

Your heart is pounding out of your chest.

You're sweating.

You're on the edge of a stroke at any moment.

Wow.

In that very specific case we use meterosine to cut the supply line to shut down the runaway factory.

Okay that makes sense.

So that's step one on the assembly line.

Tyrosine to dopa.

What's next?

Next dopa is converted into dopamine by another enzyme called dopa decarboxylase.

Wait dopamine.

I usually think of dopamine as the reward chemical in the brain.

You know pleasure, motivation.

What's it doing here in the sympathetic nerve?

That's a great question and in this context dopamine is actually just a precursor.

It's a work in progress.

It's not the final product yet.

It's just another step on the line.

Exactly and once dopamine is made it has to be moved immediately.

It gets transported into something called a storage vesicle.

Think of this like a protective shipping container inside the cell.

Why does it need to be in a container?

Why can't it just hang out on the factory floor?

Because the cytoplasm of the nerve of the factory floor is full of other enzymes that will destroy it if it sits out too long.

Specifically an enzyme we'll talk about later called MAO.

So it needs to be packaged to be safe.

Okay so dopamine goes into the box.

And this is where final magic happens.

Inside that vesicle there is one last enzyme called dopamine beta hydroxylase.

It converts the dopamine into norepinephrine.

So the final product is actually created inside the packaging.

That's clever.

It is very efficient.

Yeah and it sits there stored and ready to go.

Now looking at the history pharmacology which the book touches on we used to have drugs that targeted this storage step.

The text mentions reserpine.

Yeah I see that.

Reserpine.

It says it blocks the transport of dopamine into the vesicle.

Right it blocks a specific transporter called VMAT.

The vesicular monamine transporter.

So if dopamine can't get into the vesicle.

It can't turn into norepinephrine and it gets eaten up by the enzymes on the floor.

You got it.

The nerve runs out of AMMA.

So it just depletes the nerve of its transmitter.

Effectively yes.

Yeah.

Reserpine was used for hypertension and even psychosis way back in the day.

But it's rarely used now because it's a bit of a sledgehammer.

It's not subtle.

Why is it?

Because it depletes these chemicals in the brain too.

Not just in the periphery.

And that can cause some severe side effects.

Most notably depression.

Okay so let's assume everything is working normally.

We have norepinephrine sitting in the vesicle packaged up and waiting.

What causes it to be released?

An action potential.

An electrical signal zips down the nerve.

When it hits the terminal it triggers a voltage gated calcium channels to open.

Calcium rushes into the cell.

Calcium seems to be the trigger for almost everything in the body.

It really is.

It's the universal messenger.

In this case the calcium causes the vesicles to move to the edge of the cell membrane, fuse with it and spill their contents out into the synaptic cleft.

So they just dump the norepinephrine out.

We call it calcium mediated exocytosis.

Yeah.

Yeah they dump it out.

So now the norepinephrine is out in the wild.

It's in the floating across to hit the receptors on the other side.

But the text mentions a safety valve here.

It talks about negative feedback.

This is a crucial concept.

On the nerve terminal itself, the one that's sending the signal,

there are special receptors called alpha two receptors.

So these are on the sender not the receiver.

Correct.

They are pre -synaptic.

We call them auto receptors.

Think of them like a sensor on a gas pump nozzle.

When the tank is full the sensor trips and stops the flow.

Oh okay.

So when norepinephrine fills up the synapse, some of it floats back, hits this alpha two receptor on the nerve that released it and tells the nerve, okay that's enough.

Stop releasing more.

Exactly.

It inhibits further calcium influx and stops more vesicles from fusing.

It's a beautiful built -in braking system.

And I imagine there are drugs that mess with this.

Oh absolutely.

We'll see later that some very important drugs like clonidine actually hack this specific system to lower blood pressure.

Fascinating.

Okay so we've made it, we've stored it, and we've released it.

The signal has been sent.

Now how do we clean up the mess?

We can't leave the signal playing forever.

Right.

We need termination of action.

There are two main ways the body clears the synapse.

The first and by far the most important is reuptake.

The vacuum cleaner.

Yes.

That's a great way to put it.

There is a specialized pump protein called catecholamine transporter,

or more specifically the norepinephrine transporter, NET.

It physically sucks the norepinephrine back into the nerve terminal it came from.

To be destroyed.

Or recycled.

It's very efficient.

About 75 % or more of the norepinephrine is sucked back up, put back into vesicles, and used again.

Very green.

And this transporter is important for pharmacology because.

I feel a big one coming.

Because this is exactly where drugs like cocaine and certain antidepressants work.

Cocaine blocks this transporter.

It jams the vacuum cleaner.

So the norepinephrine just stays out in the synapse.

It stays out there bouncing around, hitting the receptors over and over and over again.

It can't be cleared.

That's why you get that massive surge of adrenaline, that energy with cocaine.

You've broken the cleanup mechanism.

Wow.

Okay.

What about the norepinephrine that doesn't get recycled by the vacuum, the other 25 %?

That gets handled by the second mechanism,

enzymatic metabolism.

It's the incinerator crew.

There are two enzymes you absolutely need to know.

MAO and COMT.

MAO stands for monoamine oxidase.

Right.

MAO is mostly located inside the neuron itself on the mitochondria.

It cleans up the norepinephrine that might leak out of the vesicles or gets pumped back in but doesn't get repackaged.

It's the internal housekeeper.

And QMT.

QMP is

catecholomethyltransferase.

COMT is mostly found outside the neuron in other tissues like the and the gut wall.

It's responsible for breaking down catecholamines that are circulating in the blood.

This explains why we can't take certain drugs orally, doesn't it?

Spot on.

If you swallowed a pill of pure epinephrine, the COMT in your gut lining and liver would chew it up before it ever reached your heart or your lungs.

It would be useless.

That's why epinephrine has to be injected, like in an EpiPen.

Precisely.

Okay.

That is the full life cycle.

Factory, storage, release, auto receptor and cleanup.

That is the biological stage we are playing on.

It is.

And before we throw drugs onto that stage, we need to understand one major reflex arc that the body has.

The text highlights this in section 2, the baroreceptor reflex.

This part always trips people up because it makes the drug effects look wrong on paper if you don't account for it.

It absolutely does.

Here's the scenario.

The body hates sudden changes in blood pressure.

It wants stability, what we call homeostasis.

To maintain that, we have pressure sensors,

baroreceptors, located in the aortic arch and in the carotid sinus right in your neck.

They act like a thermostat for pressure.

More like a pressure valve.

And they don't sense pressure directly, they sense stretch.

If your blood pressure spikes,

the walls of those big arteries stretch and these receptors start firing like crazy.

They send an urgent signal up to the brain stem.

Urgency.

Pressure too high.

Exactly.

The vasomotor center in the brain stem processes this and decides to lower the pressure.

It does two things.

One, it turns down the sympathetic nervous system.

It lifts its foot off the gas.

And two, it ramps up the parasympathetic nervous system that hits the brain.

And the main parasympathetic nerve that goes to the heart is the vagus nerve.

Correct.

So a spike in blood pressure leads to a strong vagal signal, which directly slows the heart rate.

We call this reflex bradycardia.

So let's apply this.

If I give a patient a drug that squeezes their blood vessels tight of vasoconstrictor, their blood pressure goes way up.

The drug itself isn't touching their heart, but their heart rate drops.

Right.

The direct effect of the drug is on the vessels.

The reflex effect, the body's reaction, is on the heart.

And the opposite works too.

Yes.

If a drug suddenly dilates your vessels and your pressure crashes,

the baroreceptors stop stretching.

They go quiet.

The brain panics.

The brain panics and screams for more output.

It cuts the vagal break and stomps on the sympathetic gas.

Your heart rate shoots up.

Reflex tachycardia.

This is so important for students to get.

If you see a chart saying drug X lowers heart rate, you have to ask, does it lower heart rate directly because it touches the heart?

Or does it lower heart rate reflexively because it raised blood pressure so much?

Context is everything.

You cannot interpret heart rate changes without looking at blood pressure changes at the same time.

You get the question wrong every time if you do.

Okay, we have a life cycle.

We have the reflex.

Now we need the dashboard.

Section 3, the receptor dashboard.

This is where we get into the nitty gritty of alpha and beta.

And we need to go a bit deeper than just alpha squeezes.

We need to understand the signal transduction, the how.

How does the message get from the outside of the cell to the inside?

Let's start with the alpha receptors.

Alpha 1.

Alpha 1 is the workhorse of the sympathetic system when it comes to blood pressure.

It is primarily found on vascular smooth muscle, the muscles wrapped around your arteries and veins.

And when it gets activated, we get vasoconstriction.

But how does a chemical on the outside make a muscle squeeze on the inside?

What's the chain of command?

It uses a middle band called a G protein.

Specifically for alpha 1, it's a GQ protein.

GQ?

Sounds like a fashion magazine.

Laughs a little bit.

But GQ is aggressive.

When norepinephrine hits the alpha 1 receptor, it wakes up GQ.

GQ then activates an enzyme called phospholipase C.

Okay.

Phospholipase C.

And that does what?

It rips a phospholipid right out of the cell membrane to create a second messenger called IP3.

So it's an internal signal now.

Yes.

And IP3 swims through the cell to the calcium storage tank, the sarcoplasmic reticulum, and basically unlocks it.

So we get a flood of calcium inside the cell.

A huge flood of intracellular calcium.

And in muscle physiology, calcium equals contraction.

That is the molecular reason why alpha 1 squeezes things.

It floods the muscle cell with calcium.

Okay.

So alpha 1 equals GQ, which equals calcium, which equals squeeze.

Got it.

Where else do we see alpha 1 receptors besides blood vessels?

The eye.

Specifically, the radial muscle of the iris.

When alpha 1 is hit, that muscle contracts and it pulls the pupil open.

We call that mitriasis.

So that's what's in those eye drops they give you at the optometrist to dilate your eyes.

Exactly.

Got it.

Now alpha 2, we called this the silencer earlier.

Alpha 2 works very differently.

It is coupled to a G protein.

The I stands for inhibitory.

What is it inhibiting?

It inhibits an enzyme called adenyl cyclase.

This enzyme's job is to make CAMP, which is cyclic AMP, another really important second messenger, an energy or signal molecule.

Alpha 2 shuts this enzyme down.

So CMP levels drop.

They plummet.

And when CMP drops inside the presynaptic nerve terminal, it makes it much harder to release normine pinephrine.

It prevents those calcium channels from opening.

It is the molecular brake pedal.

Okay.

So alpha 1 is excitatory using calcium.

Alpha 2 is inhibitory by decreasing CAMP.

Now let's do the betas.

The beta receptors, that's beta 1, beta 2, and beta 3 are all coupled to G's proteins.

The S stands for stimulatory.

So they do the exact opposite of alpha 2.

The exact opposite.

They stimulate adenyl cyclase.

They cause a massive increase in CAMP inside the cell.

Let's look at beta 1 first.

The mnemonic is one heart.

Right.

Beta 1 is the dominant beta receptor in a heart.

When CAMP goes up in a heart muscle cell, it activates protein kinases that do a bunch of things.

But most importantly, they open calcium channels.

More calcium again.

More calcium, but through a different pathway.

And here more calcium means a stronger, faster contraction.

So beta 1 increases everything about the heart of function.

It increases the rate, which is chronotropy.

It increases the force of the squeeze, which is inotropy.

And it increases the speed of electrical conduction, dramotropy.

It just turns the pump up to 11.

Okay.

And beta 2, two lungs.

Beta 2 is dominant in the smooth muscle of the bronchioles.

But here is the paradox.

Increased CAMP in smooth muscle, like in the lungs and certain blood vessels, actually causes relaxation.

Wait, hold on.

In the heart, more CAMP causes more contraction.

In the lungs, more CAMP causes relaxation.

How can that be?

It's all about the downstream machinery that CAMP talks to inside the cell.

It's different in a heart cell versus a smooth muscle cell.

In the heart, CAMP opens calcium channels.

In smooth muscle, CAMP actually inhibits the machinery that causes the squeeze, an enzyme called myosin, the light chain kinase.

So the end result of beta 2 activation is bronchodilation.

The airways open up.

Yes.

And vasodilation.

Beta 2 is also found on the blood vessels that feed your skeletal muscles.

This makes perfect sense for fight or flight.

You want the heart pounding.

That's beta 1.

You want the airways wide open to get oxygen.

That's beta 2.

And you want to send more blood to your legs so you can run.

That's also beta 2 vasodilation.

Exactly.

Meanwhile, alpha 1 is clamping down on the vessels to your gut and skin because, you know, you don't need to be digesting a sandwich while you're running from a bear.

It is an incredibly elegant system when you lay it all out.

What about beta 3?

That one's less common.

Beta 3 is a bit of a newcomer in terms of having clinical drugs that target it.

It's found in adipose tissue, fat tissue, where it triggers lipolysis, breaking down fat for energy.

Sure.

For the running.

Right.

But clinically,

its biggest role that we target is in the bladder.

It relaxes the detrusor muscle, the main muscle of the bladder wall, which allows the bladder to fill more easily.

And finally, we can't ignore the dopamine receptors mentioned in the text.

Right.

Specifically, D1 receptors.

These are Gs coupled, so they increase CampaP.

They are found in the renal vascular bed.

When they're activated, they dilate the arteries going to the kidneys, which increases blood flow to the kidneys.

Okay.

That was a heavy lift on the science, but now we have the complete dashboard.

I'm going to try to summarize.

Go for it.

Alpha 1.

Squeeze for vessels in the eye.

Alpha 2.

The silencer.

The nerve break.

Beta 1.

The heart pump.

Beta 2.

Relax.

For lungs and muscle vessels.

Beta 3.

Bladder relax.

And D1.

Kidney flow.

Perfect.

Now we are ready for the drugs.

Section 4.

Direct acting agonists.

Let's start with the heavy hitters, the catecholamines.

Catecholamine is a chemical name.

It describes the structure, a catechol ring and an eminon tail.

Because of that specific structure, these drugs all share some traits.

They are incredibly potent, they have very short half lives, and they absolutely cannot be given orally.

Because COMT and MAO would just destroy them in the gut and liver.

Instantly.

These are the IV drips you see in the ICU.

Okay, let's profile them one by one like characters in a play.

First up, norepinephrine.

Norepinephrine is basically the body's natural neurotransmitter.

It's bottled up.

If you look at its receptor profile in figure 8 .6 in the book, it has a massive affinity for alpha 1 and alpha 2, and a pretty good affinity for beta 1.

It has almost zero effect on beta 2.

So it's alpha dominant, the clamp.

Precisely.

Its main job is vasoconstriction.

It dramatically increases peripheral resistance.

Cystolic and diastolic blood pressure shoot way up.

What about the heart rate?

It touches beta 1, so it should speed up, right?

It tries to.

The direct data 1 effect wants to increase the heart rate.

But remember the baroreceptor reflex?

The blood pressure spike from the powerful alpha 1 effect is so massive that the vagus nerve kicks in and slams the heart rate down to compensate.

So clinically, you often see the heart rate stay the same or even drop a little.

The reflex wins.

And the main use for this?

Septic shock.

This is the first -line vasopressor.

In sepsis, your blood vessels are pathologically dilated and floppy.

Your pressure is crashing.

Norpinephrine tightens the pipes back up to keep blood flowing to your brain and kidneys.

Okay, next character.

Epinephrine.

Epinephrine is the all -rounder.

It hits everything.

Alpha 1, alpha 2, beta 1, and beta 2.

It's essentially non -selective.

But the text says its effect is dose -dependent.

What does that mean?

This is a fascinating bit of pharmacology.

At very low doses, epinephrine seems to have a higher affinity for beta receptors than alpha.

Okay.

So if you give a tiny IV dose, you might actually see vasodilation from the beta 2 effect and a heart rate bump from beta 1.

The blood pressure might not change much or could even dip.

But at high doses?

At high doses, the alpha 1 effect completely overwhelms everything else.

You get massive vasoconstriction, just like with norpinephrine.

Let's talk about box 8 .1 in the text.

The bee sting.

Why is epinephrine the drug of choice for anaphylaxis?

Why not just use norpinephrine to fix the blood pressure?

Think about the two life -threatening problems in anaphylaxis.

One,

your blood vessels are dilating and leaking.

That's shock.

Two, your airway is swelling shut.

That's bronchoconstriction.

So you need a drug that solves both problems at once.

And norpinephrine can solve the blood pressure with its alpha 1 effect.

But it does absolutely nothing for the lungs.

It doesn't touch beta 2.

Ah, but epinephrine does.

Epinephrine hits alpha 1 to raise the blood pressure, and it hits beta 2 to blast the airways open.

It's the perfect antidote.

It's the only drug for that job.

Plus, it also stabilizes the mast cells to stop them from releasing more histamine.

It's a triple threat.

Okay, next.

Isoproterenol.

Isoproterenol is a synthetic drug.

It is pure beta.

It powerfully hits beta 1 and beta 2, but has zero affinity for alpha receptors.

Zero alpha.

So what does that look like in a patient?

The beta 1 effect means the heart goes crazy rate and squeeze go way up.

The beta 2 effect means all the skeletal muscle blood vessels dilate wide open.

The peripheral resistance plummets.

So systolic pressure might go up a bit because of the strong heart pump, but diastolic pressure must crash because the vessels are so open.

Exactly.

The mean arterial pressure actually falls.

It's a very specific and frankly a very stressful profile for the heart.

We don't use it much anymore, but you might still see it used for severe refractory bradycardia, or heart block to just chemically kickstart the heart.

How about dopamine?

Dopamine is the chameleon.

Its effect changes completely depending on how fast you run the IV pump.

It's all dose dependent.

Okay, walk us through the gears.

Low dose first.

At a low dose,

around one to two mikes per kilo per minute, it primarily hits D1 receptors.

Dekinia receptors.

Right.

It dilates the renal arteries.

Historically, people use what they call renal dose dopamine to try to protect the kidneys in patients in shock, though the data on whether that actually works is pretty controversial now.

Medium dose.

In the medium range, say five to 10 mikes, it starts hitting beta -1 receptors.

Now you get increased heart contractility and inotropic effects.

At high dose.

At high dose, over 10 mikes, it starts hitting alpha -1.

Now it just acts like more pyphery and you get vasoconstriction.

So you'd be very precise with your dosing to get the effect you want.

Last catecholamine on the list, dubutamine.

Dubutamine is structurally similar to dopamine, but its pharmacology is much cleaner.

It's a relatively selective beta -1 agonist.

Cardioselective.

You call it that, yes.

It focuses on increasing contractility, the inotropy, more than it increases the heart rate.

It strengthens the squeeze without making the heart rates uncontrollably.

So this is for heart failure.

Acute heart failure.

When the pump is failing and you just need to give it a temporary boost to move fluid forward and improve cardiac output.

Before we leave this powerful section, what are the dangers?

These seem like high stakes drugs.

They are.

The biggest local risk is extradition.

This is a nightmare scenario.

If the IV line slips out of the vein and starts pumping norepinephrine into the surrounding skin and fat.

The alpha -1 effect kicks in locally.

You get intense, complete alpha -1 vasoconstriction in that patch of skin.

It cuts off all the blood supply.

The tissue dies.

It dies.

You get necrosis.

If that happens, the textbook says you have to immediately infiltrate the area with an alpha blocker, like phentalamine, to try and reverse the clamp and save the tissue.

And systemically, what are the risks?

Arrhythmias.

You are whipping the heart.

You can easily trigger dangerous rhythms like ventricular tachycardia.

And particularly with epinephrine, if the blood pressure spikes too fast or too high, you can cause a cerebral hemorrhage, a stroke.

Okay, so these are not to be taken lightly.

Not at all.

Let's dial it down a notch.

Section 5.

The non -catecholamines.

These are the drugs that don't have that vulnerable catechol ring structure.

Which means two big things.

Yeah.

They can be taken orally, and they last much longer because COMT can't break them down.

These are your outpatient drugs.

Let's start with phenylephrine.

I definitely recognize this from the cold and flu aisle.

Phenylephrine is a pure alpha -1 agonist.

It does one thing and one thing only.

It squeezes vessels.

So how does squeezing vessels help a runny nose?

Well, congestion isn't really snot.

It's swollen blood vessels in the nasal mucosa.

They get dilated and leaky.

Phenylephrine constricts those nasal vessels, which shrinks the tissue and opens up the breathing passage.

So it's a decongestant.

It's a classic decongestant.

But we also use it intravenously in the hospital, often in the operating room, to raise blood pressure during anesthesia.

It's useful because it raises BP without speeding up the heart since it has no beta activity at all.

Then there's midadrine.

Sounds similar.

Also a pure alpha -1 agonist.

Midadrine is interesting because it's a pro -drug.

It has to be activated in the liver first.

It's used for chronic orthostatic hypotension.

This is for the patient who gets dizzy and passes out when they stand up?

Exactly.

Their autonomic system is too slow or broken to clamp the vessels when they stand up.

Midadrine provides a baseline clamp.

It keeps the vascular tone up so blood can get to the brain against gravity.

Makes sense.

Now let's switch gears to the lung drugs.

Albuterol, sameterol, tobutylene.

These are our beta -2 selective agonists.

Their whole job is to relax the bronchial smooth muscle.

Albuterol is the rescue inhaler.

Yes, it's short -acting.

You use it for an acute asthma attack.

Salmeterol is long -acting.

It's used for maintenance control in conditions like COPD and more severe asthma.

You mentioned selective.

Does that mean they have zero side effects?

Selective is always a relative term in pharmacology.

If you take one or two puffs of an albuterol inhaler, it mostly hits the beta -2 receptors in your lungs.

But if you take 10 puffs or use a nebulizer or swallow a tablet, some of that drug is going to spill over and hit the beta -1 receptors in the heart.

And that's why patients get the jitters, the tremors, and the tachycardia.

That's exactly why.

The text has a specific warning about tobutylene, particularly regarding pregnancy.

Yes, because the uterus is also made of smooth muscle.

Beta -2 agonists can relax it.

So tobutylene was historically used off -label to stop premature labor.

We call that tocolysis.

It stops the contractions.

But the FDA put a black box warning on it?

They did.

They advised against using it for more than 48 to 72 hours.

Prolonged use puts a massive stress on the mother's cardiovascular system.

We're talking pulmonary edema, severe tachycardia, even death.

It's a temporary fix, not a long -term treatment.

I mean, let's move to the imidazolines.

These are the drugs ending in zolene or iodine.

Right.

First, we have oxymethasoline.

That's your Afrin nasal spray.

It's a long -acting alpha agonist.

It works just like phenylephrine, but lasts for 12 hours.

And the big warning here is about rebound congestion.

This is a classic board exam question.

If you use a spray like Afrin for more than three days in a row, your alpha receptors downregulate.

They get used to the constant stimulation.

So they become less sensitive.

Right.

Then when you stop the spray, the vessels dilate even worse than they were before.

You feel more congested, so you use the spray again.

You get hooked on it.

The official name is rhinitis medicamentosa.

That's the one.

So three days max is the rule.

Now, clonidine.

This one always feels counterintuitive.

It's an adrenergic agonist.

It activates the system, but we use it to lower blood pressure.

This brings us right back to that alpha -2 auto receptor we talked about at the beginning.

Clonidine is an alpha -2 agonist, but it works primarily in the central nervous system, in the brainstem.

So it goes into the brain and stimulates the brake pedal.

Yes.

It tricks the brainstem into thinking there's way too much sympathetic activity going on.

So the brain's response is to reduce sympathetic outflow to the rest of the body.

It effectively unplugs the whole system at the source.

Pretty much.

So it lowers blood pressure and heart rate.

But because it dampens all that sympathetic noise, it's also used for other things, like helping with focus and ADHD and for calming the terrible withdrawal symptoms from opioids or alcohol.

It calms the storm.

There are a couple of others in this group.

Dexmedetomidine.

Or Presodex.

It's another alpha -2 agonist, used almost exclusively as a sedative in the ICU.

It's unique because it sedates the patient without depressing their breathing, unlike fentanyl or propofol.

You can actually wake the patient up to talk to them, and they'll go right back to a calm, sedated state.

And to Xanadide.

Same mechanism, but it works primarily on the spinal cord to reduce muscle spasticity.

It's a muscle relaxant, good for MS or cerebral palsy.

And before we wrap the direct actors, let's hit the outliers.

Mirabegron.

That's our one and only beta -3 agonist.

Its job is to relax the bladder wall, the detrusor muscle.

So it's used for overactive bladder.

It increases the bladder's capacity to hold urine.

And Draxodopa.

Draxodopa is cool.

It is a synthetic form of L -dopa.

You swallow it as a pill.

It crosses into the brain and nerves.

And it gets converted by the body's own enzymes into norepinephrine.

So you're basically giving the factory more raw material.

Exactly.

You're using the body's own machinery to make the drug where it's needed.

It's used to treat neurogenic orthostatic hypotension.

Okay, section six.

We have covered the direct agonists.

Now we have the indirect and mixed acting agonists.

These are the tricksters.

They don't touch the receptor directly.

They manipulate the supply of norepinephrine.

Right.

The indirect agents work by cranking up the concentration of norepinephrine that's already in the synapse.

Let's start with the big one.

Amphetamine.

Amphetamine is a classic indirect acting adrenergic agonist.

It does two very aggressive things.

First, it's able to get inside the nerve terminal and force its way into the storage vesicles.

And it physically displaces the norepinephrine, kicking it out into the cytoplasm.

So it kicks the workers out of the break room.

It does.

And then for its second trick, it reverses the direction of the membrane transporter, the net, the vacuum cleaner.

Instead of sucking norepinephrine in for recycling, the pump starts running in reverse, vomiting norepinephrine out into the synapse.

It creates a flood.

A massive flood of both norepinephrine and dopamine.

This is why it causes that intense alertness, focus in euphoria, but also why it's so dangerous, causing severe hypertension and tachycardia.

Okay, now compare that mechanism to cocaine.

Cocaine is purely a reuptake inhibitor.

It's more passive.

It doesn't reverse the pump.

It just blocks it.

It puts a cork in the vacuum cleaner.

Norepinephrine is released normally, but it can't leave the synapse.

So it just accumulates.

So a different mechanism, but a similar end result of too much neurotransmitters.

A very similar result.

The text also notes cocaine is a local anesthetic.

It blocks sodium channels.

But clinically, the reason we respect cocaine so much in the ER is because of that intense vasoconstriction.

It can cause heart attacks in healthy 20 -year -olds because it can clamp the coronary arteries completely shut.

Scary stuff.

What about tiramine?

You mentioned the cheese effect earlier.

Tiramine is a compound found in a lot of fermented foods.

Aged cheese, cured meats, red wine, soy sauce.

Normally, if you eat some cheese, the MAO enzyme in your gut wall destroys the tiramine immediately.

No problem at all.

But if you are on an MAO inhibitor...

If you're taking an MAOI, an older class of antidepressant, that MAO enzyme in your gut is disabled.

The tiramine gets absorbed into your blood, reaches your sympathetic nerves, and acts just like amphetamine.

That causes a massive release of all your stored norepinephrine at once.

Boom, hypertensive crisis.

A catastrophic one.

Your blood pressure can go to 220 over 120.

That's stroke territory.

So it's why patients on MAOIs have to follow a very strict tiramine -free diet.

There is a new drug mentioned here, Solrium Fetal.

Yes, it's a DNRI, a dopamine and norepinephrine reuptake inhibitor.

So it inhibits the transporters like cocaine, but it doesn't cause that massive release like amphetamine.

It's a bit more controlled.

It's used for narcolepsy and excessive sleepiness and sleep apnea to promote wakefulness.

Okay, finally, the mixed acting agents, ephedrine and pseudoephedrine.

Mixed means they do both.

They have some indirect action.

They stimulate the release of stored norepinephrine like amphetamine anvrin.

They can also directly bind to alpha and beta receptors themselves like epinephrine.

Pseudoephedrine is pseudephed.

But the real stuff, the stuff you have to get from behind the pharmacy counter.

It's an excellent decongestant because of that mixed vasoconstriction effect, but it is chemically very, very similar to methamphetamine.

Which is why you have to show your ID to buy it.

Exactly.

The Combat Methamphetamine Epidemic Act was passed because it's a common precursor for illicit meth production in home labs.

What about side effects for pseudephed?

I feel like people treat it like it's just candy.

It's not.

It can definitely raise your blood pressure.

It causes insomnia because it gets into the brain and acts as a stimulant.

And this is a crucial point for older men.

It can cause urinary retention.

Why is that?

Alpha -1 receptors are on the sphincter of the bladder.

Pseudephed with its direct alpha agonist activity clamps that sphincter shut.

If a man already has an enlarged prostate, BPH, pseudephed can make it completely impossible for him to urinate.

It's a medical emergency.

That is a great clinical pearl.

Okay, we have covered the entire landscape.

We've built the factory, we've mapped the receptors, and we've categorized all the drugs.

Section seven, review and synthesis.

I want to throw a few of the textbooks review questions at you just to see if we can apply all this.

I'm ready.

Let's do it.

Okay, question two concept from the book.

A man is given a drug that inhibits the synthesis of norepinephrine.

So something like meterecine.

What would be the expected physiological result?

The options include things like bronchodilation, diarrhea, et cetera.

Let's just reason it out.

If you inhibit norepinephrine synthesis, you are turning down the entire sympathetic system.

You are removing the fight or flight influence from the body.

So the rest and digest system, the parasympathetic system runs unopposed.

Exactly.

So you just ask, what does the parasympathetic system do?

What does it do to the gut?

It increases motility.

So diarrhea would be an expected effect.

What does it do to the heart?

It slows it down.

So bradycardia.

Makes perfect sense.

Okay, question three concept.

A patient with diabetic neuropathy gets dizzy every time he stands up.

Orthostatic hypotension.

Which of the drugs we talked about would help?

The problem is his sympathetic reflex is broken.

He can't clamp his vessels when he stands.

So we need to give him a drug that does it for him.

We need to clamp the vessels.

We need an alpha -1 agonist.

Mitadrine is the classic answer here.

It provides that constant vascular tone he's missing.

Last one, question six concept.

We give epinephrine for a bee sting.

Which specific molecular mechanism relieves the bronchoconstriction?

Okay, so the goal is bronchodilation.

That happens in the lungs.

The receptor in the lungs is beta -2.

The beta -2 receptor, we said, is coupled to Gs.

G stimulates adenyl cyclase, which increases intracellular CMP.

So the answer is a beta -2 mediated increase in CMP.

You nailed it.

It's all about the map.

It all connects.

Once you understand the receptor location and the signal pathway, you don't have to memorize every single fact.

You can derive the answer from the principles.

So bringing it all together for the outro.

What is the big takeaway here?

What should students really hold on to from chapter eight?

For me, it's about the concept of precision versus balance.

The sympathetic nervous system is, by design, a blunt instrument.

It floods the whole body with a signal to save your life.

Pharmacology is our attempt to refine that instrument.

We want the squeeze on the blood vessels, but maybe without the heart attack.

We want the open airway without the tremors and the racing heart.

Exactly.

And understanding the subtle differences.

Why dobutamine is different from dopamine or why clonidine lowers blood pressure instead of raising it.

That is the art of medicine.

It allows us to tune the human engine rather than just hitting it with a hammer.

And it's a reminder that these are incredibly powerful tools.

As we saw with the extravasation example or the MAOI cheese interaction,

the biology is potent and unforgiving if you don't respect it.

Respect the receptor.

That should be the motto for this chapter.

I have a final thought on that.

It's fascinating how the same mechanism can be both a cure and a poison.

How so?

That alpha 1 vasoconstriction.

In a patient dying of septic shock, it's the most life -saving thing in the world.

But that same exact mechanism from an IV that slips into the skin of a forearm can kill the tissue and cause them to lose a limb.

The dose, the location, the context.

It's everything.

That's a profound way to put it.

The difference between a cure and a poison is pharmacology.

Well, I feel like I finally understand Chapter 8.

Thank you for walking us through the factory floor.

My pleasure.

It really is a foundational chapter.

Everything else in cardio and pulmonary pharmacology builds on this.

We hope this deep dive helped you build that mental framework.

Visualize the vesicle.

Visualize the G proteins.

And we will see you on the next one.

Good luck with your studies.

This has been the Last Minute Lecture Team.

Signing off.